Spring源码分析：starter

01｜自定义Starter

1.1 引入依赖

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <groupId>white.colde</groupId>
    <artifactId>spring-starter</artifactId>
    <version>0.0.1</version>
    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-autoconfigure</artifactId>
        </dependency>
    </dependencies>
    <dependencyManagement>
        <dependencies>
            <dependency>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-dependencies</artifactId>
                <version>2.4.1</version>
                <type>pom</type>
                <scope>import</scope>
            </dependency>
        </dependencies>
    </dependencyManagement>
</project>

1.2 创建业务类

public interface LogService {
    void log(String log);
}
public class LogServiceImpl implements LogService {
    @Override
    public void log(String log) {
        System.out.println("log:"+log);
    }
}

1.3 创建配置类

@Configuration
public class LogServiceAutoConfiguration {
    @Bean
    public LogService getLogService(){
        return new LogServiceImpl();
    }
}

1.4 创建配置文件

在resources目录下创建META-INF文件夹，并创建spring.factories文件，用于声明配置类完整路径

org.springframework.boot.autoconfigure.EnableAutoConfiguration=xxxx.LogServiceAutoConfiguration

1.5 使用

在所需项目中引入starter依赖，即可使用starter中所定义的bean对象

02｜starter自动化配置原理

依据自定义starter的流程，可以大致猜测Spring自定义Starter的思路应该如下：

• 1、扫描导入所有包中的spring.factories文件
• 2、解析出配置文件中的Spring配置类
• 3、解析配置类，注入配置类中定义的所有Spring Bean并加入到IoC容器中

在1的starter的示例中，最终在spring.factories中声明了一对属性配置

org.springframework.boot.autoconfigure.EnableAutoConfiguration=xxxx.LogServiceAutoConfiguration

打开EnableAutoConfiguration的源码可以发现，EnableAutoConfiguration是一个注解，并在EnableAutoConfiguration注解之上导入了AutoConfigurationImportSelector类

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {
    String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";
    Class<?>[] exclude() default {};
    String[] excludeName() default {};
}

查看AutoConfigurationImportSelector类，从方法名就可以看出selectImports方法应该就是我们所找的方法，跟踪方法，最终会调用getCandidateConfigurations方法，并在方法中调用了SpringFactoriesLoader.loadFactoryNames方法载入了所有jar包中的的spring.factories中的配置类信息

public class AutoConfigurationImportSelector implements DeferredImportSelector, BeanClassLoaderAware,
        ResourceLoaderAware, BeanFactoryAware, EnvironmentAware, Ordered {
    @Override
    public String[] selectImports(AnnotationMetadata annotationMetadata) {
        if (!isEnabled(annotationMetadata)) {
            return NO_IMPORTS;
        }
        AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(annotationMetadata);
        return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
    }
    protected AutoConfigurationEntry getAutoConfigurationEntry(AnnotationMetadata annotationMetadata) {
        if (!isEnabled(annotationMetadata)) {
            return EMPTY_ENTRY;
        }
        AnnotationAttributes attributes = getAttributes(annotationMetadata);
        List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);
        configurations = removeDuplicates(configurations);
        Set<String> exclusions = getExclusions(annotationMetadata, attributes);
        checkExcludedClasses(configurations, exclusions);
        configurations.removeAll(exclusions);
        configurations = getConfigurationClassFilter().filter(configurations);
        fireAutoConfigurationImportEvents(configurations, exclusions);
        return new AutoConfigurationEntry(configurations, exclusions);
    }
    protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {
        List<String> configurations = SpringFactoriesLoader.loadFactoryNames(getSpringFactoriesLoaderFactoryClass(),
                getBeanClassLoader());
        Assert.notEmpty(configurations, "No auto configuration classes found in META-INF/spring.factories. If you "
                + "are using a custom packaging, make sure that file is correct.");
        return configurations;
    } 
}

最终会将解析出的配置信息注册到beanFactory中，等待IoC容器进行实例化